The present invention relates to cellular telephone systems, and more particularly to a portable, hand-held, cellular telephone having a radio frequency (rf) switch that switches its rf output signal between two rf signal paths in order to achieve a desired type of modulation, and in order to conserve power.
Cellular telephones have been widely used in recent years in automobiles and other mobile vehicles where there has been available an adequate power source to power the operation of the cellular phone. Until quite recently, however, portable cellular telephones (cellular phones that could be removed, carried away from a main power source and operated for both reception and transmission) required the user to also carry a relatively heavy and bulky "battery pack", resembling a large handbag or briefcase. Needless to say, such "portability" left much to be desired.
Within the past couple of years, or so, the hand-held, portable cellular telephone has emerged. A hand-held portable cellular telephone fits comfortably in the hand of a user, may be conveniently carried in a pocket or handbag, and resembles a conventional telephone handset. It includes a mouthpiece or microphone into which the user can speak, and an earpiece or speaker to which the user can listen, while holding the unit in his or her hand. Advantageously, all of the circuitry required for both transmission and reception of cellular telephone signals is included within the housing of the portable cellular telephone, including a battery that is sufficiently small and light weight to allow the hand-held phone to be comfortably held in the user's hand.
Disadvantageously, battery technology has not yet advanced sufficiently to allow a battery of a hand-held portable cellular telephone to power its operation for more than about 10-14 hours when receiving cellular signals, and only about 1 hour or so for transmitting cellular signals. As a result, users of hand-held portable cellular telephones must constantly recharge the battery or batteries of the cellular unit, and/or have replacement batteries readily on hand. There is thus a need in the art for a hand-held cellular telephone that can operate over a longer period of time between battery charges.
One advance that has recently occurred in cellular telephone technology is the availability of various digital modulation schemes that allow more cellular signals (which are modulated radio frequency, or "rf" signals) to be transmitted over the available air waves, or channels, than has previously been possible using conventional FM modulation. Such digital modulation schemes are particularly useful to transfer all of the information needed to track and maintain contact between a portable, or moving, cellular telephone and the nearest stationary transceiver location. However, in order to maintain compatibility with previous (earlier) cellular systems and telephones, there remains a need to retain FM modulation capability. Hence, it is known in the cellular telephone art to provide a dual mode cellular telephone, i.e., one that can selectively operate using a digital modulation scheme or a conventional FM (frequency modulation) scheme.
A dual-mode portable cellular telephone requires, in effect, two rf paths in its output stage, one path for each type of modulation that is available. Having dual rf paths is of little consequence when there are no power constraints or space limitations associated with the cellular telephone because two separate and independent rf channels can be provided internal to the telephone. Unfortunately, a portable hand-held cellular telephone has both power constraints and space limitations, and the use of two independent rf signal paths causes more bulk in the size of the telephone and more power consumption than would otherwise be needed if only one rf signal path were provided. Thus, it would be desirable to have a hand-held, dual mode, portable cellular telephone wherein the need for separate, independent rf paths in the output stage were eliminated.
Many elements included in the rf path through the output stage of a cellular telephone are the same, regardless of the type of modulation that is provided. Such common elements, such as rf amplifiers, attenuators, power amplifiers, and the like, could be shared between both rf signal paths if there were only a convenient way to switch the rf signal path to include the desired type of modulator. Unfortunately, while mechanical rf switches are well known in the art, the reliability of such switches is suspect, making their use unsuitable for a cellular telephone of high reliability. Further, such switches tend to be quite bulky. Hence, there is also a need in the art for a non-mechanical, reliable, rf switch that allows the rf signal path through the output stage of a cellular telephone to be readily toggled to provide one type of modulation or another, as required.
The present invention advantageously addresses the above and other needs.